The invention relates to optical devices and techniques for sensing properties of an environment, such as the presence of a substance in the environment or the measurement of pressure, magnetic fields or electric fields.
The development of integrated optical devices has evolved in recent years, and these devices have shown promise of providing highly sensitive detectors of substances in an environment, such as chemicals or biological materials. The devices also measure other properties, such as pressure, magnetic fields or electric fields. A class of these devices, which are of interest herein, measure a change in index of refraction caused by the property being measured. The light which is used to determine the change in index of refraction can be carried to and from the device by optical fibers, so the devices can be of a passive nature that is advantageous for many applications, for example monitoring explosive mixtures, flammable materials or toxic substances, etc. The devices can be very small in size and can be put in relatively inaccessible places. The devices are also relatively inexpensive to manufacture in quantity, and a number of them can be utilized together to obtain measurements that are over a wide range of values as well as highly sensitive.
An example of the described type of device is shown in the U.S. Pat. No. 4,515,430 of Johnson which discloses an integrated optical transducer that is stated to include a single mode input optical waveguide formed in a single substrate which divides into optical waveguide branches of different physical lengths, the two branches recombining into a single mode output waveguide. When used as a transducer, the optical path lengths of the waveguide branches are stated to be dependent on a physical quantity to be measured. The patent notes that the transducer can be used as a temperature sensing transducer or may be used to sense other parameters on which either the index of refraction or the length of the waveguide branches are dependent, including pressure, strain, electric or magnetic fields, electromagnetic radiation, or chemical activity.
Notwithstanding the listed potential advantages of previously proposed integrated optical sensors such as the one set forth in the Johnson patent, there are disadvantages which can limit their practical application. Because there are separate optical paths for the respective branches, temperature changes can effect the branches differently, particularly when the branches are of different lengths as specified in the Johnson patent. Therefore, temperature variations can introduce substantial errors when attempting to measure properties such as the presence of a substance, presence of a field, etc. The two arms may also be effected differently by mechanical perturbations. Additionally, the requirement for single mode operation, as in Johnson, renders the device fabrication more difficult and expensive.
It is among the objects of the present invention to overcome the indicated disadvantages of the prior art and to provide improvement in integrated optical sensors.